Metal burner membrane

ABSTRACT

The invention relates to a gas burner comprising a metal burner membrane having a base section ( 201 ), a closing section ( 203 ) and a transition region in between ( 202 ). The shape of the membrane is such that the smallest radius of curvature of the transition zone is smaller than the smallest radius of curvature of the base section. Furthermore the burner membrane uninterruptedly flows over from the base section through the transition region into the closing section. The advantages of such a gas burner are amongst others a large dynamic power range, an improved flame front and a low production cost.

The present application is a divisional application of U.S. applicationSer. No. 10/553,405, filed Nov. 10, 2005, which is the National Stage ofApplication No. PCT/EP2004/050205 filed on Feb. 25, 2004, which is basedupon and claims the benefit of priority from European Application No.03101079.6, filed Apr. 18, 2003, the entire contents of all of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a gas burner comprising a metal burnermembrane.

BACKGROUND OF THE INVENTION

Prior art gas burners with different shapes and different burnermembranes have been described e.g. in WO 02/44618 A1 and WO 01/79756 A1.

The first drawback of these burners is that for a given dimension, theydo not allow for a large range in output power: at low power, i.e. ifthe gasflow is low, there is a risk for flame extinguishment, and athigh powers, i.e. if the gasflow is high, there is a risk that the flameblows off. This results in the need of a range of burners that differonly slightly in dimensions (e.g. in their height) adapted to specificpower ratings: a second drawback.

A third drawback of these burners is that different parts have to bepunched, formed and welded together which leads to expensive burners.

The welding seams themselves are weak points in the burner, because theyare most susceptible to failure in the heating and cooling cycles thatoccur during the use of a gas burner. Hence, the weldings reduce thelifetime of the product, which constitutes a fourth drawback.

SUMMARY OF THE INVENTION

It is a general object of the present invention to eliminate thedrawbacks of the prior art burners. It is a first object of the presentinvention to provide a burner with an increased range in output power.It is a second object of the present invention to provide a burner withan increased lifetime. It is a third object of the present invention toprovide a burner with a reduced production cost. It is a fourth objectof the present invention to provide a burner with an improved flamedistribution.

A gas burner according the present invention comprises a metal burnermembrane. Geometrically this burner membrane comprises a base sectionand a closing section. The base section has a smallest radius ofcurvature R_(base). What is meant with “smallest radius of curvature”will be explained further on. The base section is connecteduninterruptedly to the closing section through a transition region: thetransition region burner membrane comprises the same elements as thebase and closing section. The transition region has a smallest radius ofcurvature r_(transition) being larger than zero and being smaller orequal to R_(base): 0<r_(transition)≦R_(base). The case in which the basesection is a plane, hence R_(base) is infinitely large, is not excluded.More preferred is: 0.02×R_(base)≦r_(transition)≦0.7×R_(base). Even morepreferred is: 0.02×R_(base)≦r_(transition)≦0.35×R_(base) There is nolimitation on the smallest radius of curvature of the closing section.

The notion of “smallest radius of curvature of a section” will now beexplained:

Geometrically, at each point of the burner membrane, many radii ofcurvature can be defined: each of them is associated with a particularcut according a plane containing the normal line at the point underconsideration. The intersection of this plane with the burner membraneresults in a trajectory. The radius of curvature is the radius of thecircle in the intersecting plane, which osculates to second order thetrajectory at the point under consideration. Out of all these possibleplanes, containing the normal line through the point underconsideration, with associated trajectories and radii of curvature, thesmallest radius is selected. As each point of a section has a smallestradius, the smallest of all smallest radii of the section can be definedto be the smallest radius of curvature of this section. As the radius ofcurvature is always a positive number, the smallest radius of curvaturethat may be found is zero. The same definition applies mutatis mutandisto each of the three parts of the burner membrane: the base section, thetransition region and the closing section. For each of them a smallestradius of curvature can thus be found. For example: for a base sectionhaving a tubular shape with a rounded polygonal cross section thissmallest radius of curvature is equal to the radius of the rounding inthe edges. Likewise for a cylinder the smallest radius of curvature isequal to half its diameter.

As this geometrical construction must be reduced to practice, it shouldbe clear that the invention relates to the embodiment of thisgeometrical construction, which of course is subject to engineeringtolerances. Hence, it should be clear that the invention is notdelimited to the abstract geometrical shape as such but to the shape ofthe actual burner membrane. This shape can be easily measured by meansof an appropriate computerised 3-D measuring bench that allows forimmediate determination of the geometrical features in general and theradii of curvature in particular.

The shape of the burner membrane influences the functioning of theburner in the following way: those regions of the burner membrane thathave a smaller radius of curvature yield a lower gas speed outside themembrane compared to the regions with a higher radius of curvature. Alower gas speed leads to a lower flame front. So the speed of the gasoutside the membrane, and subsequently the flame front, can beadvantageously modulated over the surface by changing the radius ofcurvature.

This yields, amongst others, the following advantages:

-   -   Due to the area of reduced gas speed, the flame is less prone to        blow-off.    -   Due to the different gas speeds over the burner membrane, a        large variation in gas flow rate can be accommodated with the        same burner, thus eliminating the need to have different types        of burners on stock.    -   The area with a smaller radius of curvature, due to the slower        gas flow, lends itself advantageously for the ignition of the        gas.

According to the present invention the transition from base section toclosing section is realised without interruption. With uninterrupted ismeant that the membrane forming the different sections (base, transitionand closing) are not connected by any means that would lead to a seam ofthe membrane with a blocked gas flow at the burner surface as a result.I.e. the three sections: base, transition and closing must be gaspermeable. The fact that the burner membrane is free of interruptionensures a closed flame front throughout the whole burner membrane. Thethree sections (base, transition and closing) can be realiseduninterruptedly in one of the following ways:

-   -   by using a fabric of braided or knitted or woven stainless steel        fibres. Such fabric can be woven or braided or knitted in such a        way that it fulfils the geometrical requirements of the        invention;    -   by deep drawing or stamping a plate into a shape which fulfils        the geometrical requirements of the invention. Small holes must        be drilled into the plate in the three sections (base,        transition and closing) in order to achieve the desired gas        flow;    -   by deep drawing or stamping of an already foraminated plate thus        eliminating the need for drilling holes into the plate        afterwards;    -   by deep drawing or stamping a wire mesh where the wires have a        suitable thickness and formability

Combinations of the above methods are possible, e.g.

-   -   a fabric of braided or knitted or woven stainless steel fibres        which is stretched over a deep drawn or stamped plate in which        holes are drilled;    -   a fabric of braided or knitted or woven stainless steel fibres        which is stretched over a deep drawn or stamped foraminated        plate;    -   a fabric of braided or knitted or woven stainless steel fibres        that is supported by a deep drawn or stamped wire mesh. The wire        mesh can also be integrated into the stainless steel fibre        fabric i.e. it can be interbraided or interknitted or interwoven        with the stainless steel fibres.

It is clear that the above enumeration is non-exhaustive and evendifferent possibilities according the claims of this invention arepossible.

By realising the burner membrane in this way, one or more of thefollowing advantages, amongst others, can be achieved:

-   -   a reduction in production cost is obtained by elimination of the        welding seams and the assembly of the different parts of the        prior art burners, by the use of a deep drawn or stamped plate        or foraminated plate;    -   an improved lifetime of the gas burner is obtained due to the        elimination of the welding seams;    -   the use of stainless steel fibres on top of the foraminated        plate isolates the flame from the plate and results in a lower        thermal stress on the foraminated plate and hence an improved        lifetime;    -   the use of stainless steel fibre results in a further random        scattering of the gas flow upon exit of the feed through holes        which leads to an improved flame distribution.

The uninterrupted burner membrane ensures a flame front in every sectionof the burner and in particular in the transition region. This improvesgreatly the stability of the flame.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIG. 1 illustrates the basic geometrical principles of the invention inperspective view.

FIG. 2 illustrates a preferred embodiment of the invention inperspective view

FIG. 3( a) shows a cut of the preferred embodiment along the line A, A′of FIG. 2 along with the geometrical elements.

FIG. 3( b) shows a cut of the preferred embodiment along the line A, A′of FIG. 2 along with the physical features.

FIG. 4 (a) shows a second preferred embodiment based on a rectangularcross section of the base section.

FIGS. 4( b) and 4(c) show the section through planes AA′ and BB′ of FIG.4( a) respectively.

FIG. 4( d) shows a top view cross section of the burner of FIG. 4( a),through the middle of the base section.

FIG. 5( a) shows a third preferred embodiment in side view.

FIG. 5( b) shows the third preferred embodiment from above.

FIG. 5( c) shows an alternate to the third preferred embodiment in sideview.

FIG. 6( a) shows a fourth preferred embodiment in side view.

FIG. 6( b) shows the fourth preferred embodiment from above.

FIG. 6( c) shows an alternate to the fourth preferred embodiment in sideview.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The basic geometrical features of the invention are illustrated in FIG.1 where a shape 100 of a burner membrane is depicted consisting out of abase section 102, a transition section 104 and a top section 106. Take‘a’ as point under consideration in which ‘a’ has its normal N to thesurface. The planes P1, P2 and P3, all containing the normal N, cut thesurface of the burner along different trajectories T1, T2 and T3respectively. The osculating circle C touches T1 in ‘a’. It will beclear that of all planes containing N, the plane P1 determines thetrajectory T1 with the smallest radius of curvature R(a) at ‘a’. If nowfor every point ‘x’ (not indicated on FIG. 1) of the transition sectionthis R(x) is determined, the smallest value of all R(x)'s can be chosen.When the procedure is applied to the base section 102 a smallest radiusof curvature ‘R_(base)’ is obtained. Similarly, a smallest radius ofcurvature ‘r_(transition)’ can be found for the transition region. It isessential to the invention that the smallest radius of curvature of thetransition region is smaller than or equal to the smallest radius ofcurvature of the base section.

FIG. 2 depicts a first preferred embodiment 200 in perspective view. Thebase section 201 is frustoconical in shape and reaches its minimumradius of curvature on the circle 204. The transition region 202 is asurface section of a torus and the closing section 203 is a flat disc.

FIG. 3 a shows the geometrical elements of the first preferredembodiment of FIG. 2 according to the line AA′. Only the outer surfaceof the surface membrane is depicted in order to bring forward thegeometrical elements. The frustoconical base section 201 has itssmallest radius of curvature at the smaller diameter side. The half topangle of the cone 326 was about 30° although 0° (a cylindrical basesection) turned out to work just as well (embodiment not shown). Highertop angles—the maximum being 90°, a flat plane—are also not excluded.All points on the circle 204 share the same minimum radius of curvatureR_(base) 328. The sphere 320 with radius R_(base) defines the largest‘smallest radius of curvature’ the transition region may have accordingto the invention. The transition region is part of the surface of atorus formed by a circle 324 that is rotated around the symmetry axis340. Hence, the radius of circle 324 determines the radius of thetransition region ‘r_(transition)’ 330. Part of a torus surface betweenthe plane of circle 204 and a plane parallel to the latter is taken asthe transition region. Let it be clear that the torus can also beconstructed by rotating an ellipse or an oval or any other roundedfigure around the axis of symmetry 340. Also the case, in which thetorus is degenerate i.e. when there is no hole in the middle, is notexcluded. This is e.g. the case in FIG. 3 a. The closing section 203 isa flat disc in this embodiment. In another preferred embodiment of thisinvention (no figure provided) the closing section is a small invertedsphere cap thus entailing a depression at the centre of the burnermembrane.

It will be clear from this embodiment that the crossover from basesection to transition region need not be smooth (with ‘smooth’ is meantcontinuous first order derivatives) but must be uninterrupted (zeroorder continuity).

FIG. 3 b depicts the physical features of the first preferred embodimentalong the cut according plane AA′ indicated in FIG. 2. 201′ indicatesthe stamped foraminated metal plate made out of a single piece of metalplate. The foraminated metal plate is provided with a number of holes.As the hole size is relatively large (1 mm for this embodiment), thechange in hole size at the transition region due to the deformation ofthe plate is not relevant to the flow speed of the gas. In order tospread the gas a piece of knitted metal fibre fabric 305 is tensionedover the base section, the transition section and the closing section.In this preferred embodiment, the fabric was attached to the foraminatedplate by means of spot welding although other means of fastening areequally well possible for example—without being exhaustive—by sewing orby stapling. In another preferred embodiment (no figure provided), thefabric was kept on the foraminated plate by means of a clamping ringthat was spot welded to the plate.

Knitted metal fibre fabric allows for a high elongation thus leading toa continuous transition from the base section to the closing section.The arrows 307, 308 and 309 indicate the velocity of the gas as it flowsout of the membrane. The lower gas velocity in the transition region 202is represented with a shorter vector 308, while the gas velocity at thebase section 201 and the closing section 203 is higher which isrepresented by a longer vector 309 resp. 307. Also the lower flame front310—where the gas ignites—and the outer flame front 313—where the top ofthe flame is—is indicated for each of the sections.

With this preferred embodiment, it was possible to achieve a maximumheating power of 40 kW/dm². A minimum heating power of 1 kW/dm² wasnecessary in order to get a stable flame. This yields an overall dynamicrange of 1:40.

In FIG. 4 a preferred embodiment is illustrated that is more suited forreplacement of a rectangular type burner. Here the cross-section of thebase section is essentially rectangular of which the edges are rounded.FIG. 4 b is a cross-section along plane AA′ of FIG. 4 a: the basesection 401 smoothly goes over into the transition region 402 whichapproximates the upper half of an ellipse with a minor half axisindicated by 406 and a major half axis indicated by 405. 407 indicatesthe osculating circle associated with the smallest radius of curvatureof the transition region. FIG. 4 c shows a cut along the line BB′. FIG.4 c shows an essentially identical shape as the AA′ cut, but here thehalf ellipse has been cut in two, and the two quarter pieces have beendisplaced the appropriate distance. FIG. 4 d shows the closing view of ahorizontal cut. The rounded corners have essentially merged into asemicircle with a radius equal to the half major axis of the ellipse asdescribed in FIG. 4 b.

Note that in this embodiment, the closing section has vanished into asingle line 408.

In a third preferred embodiment illustrated in FIGS. 5 a and 5 b theforaminated plate 201′ of FIG. 3 b was replaced by a stainless steelwire mesh 520. The diameter of the wires was 0.48 mm, with a square24/24 mesh size (24 wires per inch) in a 2/2 twilled weave. The minimumradius of curvature 506 in the transition region 502 was equal to 4 mmalthough a radius from 2 to 8 mm works equally well. The value of theminimum radius of curvature 508 of the base section 501 was 25 mm and ispreferably in the range of 30 to 45 mm. The closing section is a flatdisc 504. A knitted metal fibre fabric 512 was spot welded to the wiremesh.

An alternative to the third embodiment is depicted in FIG. 5( c). Likeparts of the burner membrane according the third embodiment areidentified with primed numbers. The transition region 502′ is in theform of a circular ridge. The top of the ridge has a radius of curvature506′, which turns out to be the smallest radius of curvature of thetransition region.

In a fourth preferred embodiment illustrated in FIGS. 6 a and 6 b, againa stainless steel wire mesh 610 was used. The base section 601 has avery large minimum radius of curvature, the transition region 602 has aminimum radius of curvature indicated by 606, while the closing sectionvanishes to a single line 604. The minimum radius of curvature of thetransition region 606 is 9 mm although values from 3 mm upward are alsopossible.

An alternative to the fourth embodiment is depicted in FIG. 6( c). Againlike parts of the burner membrane according the fourth embodiment areidentified with primed numbers. The transition region 602′ is in theform of a ridge extending substantially the length of the longitudinalburner membrane. The top of the ridge has a radius of curvature 606′,which turns out to be the smallest radius of curvature of the transitionregion. Again the closing section vanishes into a line 604′.

1. A gas burner, comprising: a metal burner membrane comprising a basesection that is a plane and a closing section, wherein the metal burnermembrane is uninterrupted and comprises a transition region forconnecting the base section to the closing section, wherein thetransition region has a smallest radius of curvature r_(transition)being larger than zero.
 2. A gas burner according to claim 1, whereinthe smallest radius of curvature r_(transition) of the transition regionis larger than 3 mm.
 3. A gas burner according to claim 2, wherein thesmallest radius of curvature r_(transition) of the transition region is9 mm.
 4. A gas burner according to claim 1, wherein the closing sectionvanishes into a line.
 5. A gas burner according to claim 1, wherein themetal burner membrane comprises a fabric comprising stainless steelfibers.
 6. A gas burner according to claim 5, wherein the stainlesssteel fibers are arranged essentially parallel into bundles.
 7. A gasburner according to claim 6, wherein the bundles are knitted, braided,or woven.
 8. A gas burner according to claim 1, wherein the metal burnermembrane comprises a foraminated plate or sheet.
 9. A gas burneraccording to claim 8, wherein stainless steel fibers are disposedoutside of the transition region, the base section and the closingsection.
 10. A gas burner according to claim 1, wherein the metal burnermembrane comprises a fabric of stainless steel fibers that is supportedby a deep drawn or stamped wire mesh, the stainless steel fibers beingbraided, knitted, or woven.
 11. A gas burner according to claim 10,wherein the wire mesh is integrated into the fabric of stainless steelfibers.